b. Semilunar (Aortic and Pulmonary) Valves. As mentioned before, the
bases of the two great arteries (the pulmonary arch and the aortic arch) begin at their
respective ventricles as short cylinders of the fibrous skeleton. Within each of these
cylinders are three cuplike cusps, which make up each semilunar valve. When the
ventricles contract (ventricular systole) and the AV valves have closed, the blood moves
out into the great arteries through the semilunar valves. When the ventricles relax
(ventricular diastole), the back pressure of the blood in the great arteries forces the
cusps of the semilunar valves to the center and seals off each opening.
10-28. NAVL OF THE HEART
a. Controls of Heart Function.
(1) Extrinsic controls. A number of cardiac nerves arise from both the
sympathetic and parasympathetic portions of the nervous system (chapter 12). The
sympathetic portion accelerates the action of the heart, while the parasympathetic
portion slows it down. These portions are both controlled by cardiovascular centers in
the medulla of the hind-brainstem. In addition, as everyone is well aware, various
emotional states can affect the actions of the heart.
(2) Intrinsic controls. Within the substance of the heart, certain fibers of the
myocardium have been transformed from contracting muscle tissue to
impulse-transmitting fibers. These are called Purkinje's fibers. Together, these fibers
provide intrinsic control for the action of the heart.
(a) The sinoatrial (SA) node is a collection of these fibers in the
interatrial septum. The SA node is often called the pacemaker of the heart because it
initiates each cycle of the contractions of the heart chambers.
(b) The atrioventricular (AV) node is another group of these fibers just
above the interventricular septum.
(c) Descending from the AV node is the bundle of His, which branches
into the right and left septal bundles. These branches pass down on either side of the
(d) Impulse begin in the SA node, pass to the AV node, and then de-
scend through the septal bundles to stimulate the myocardium of the ventricular walls to
(3) Humoral control. Apparently, some substances transported by the blood
can accelerate or slow the action of the heart. This situation is called the humoral
control of heart action.
b. Coronary Arteries. Previously, we have described the flow of blood through
the chambers of the heart. This blood, upon which the heart acts, is called functional